Water electrolysis device for homogenization of the current density

ABSTRACT

The invention relates to a water electrolysis device including a membrane-electrode assembly that includes a proton-exchange membrane and an anode active layer that includes an electrocatalytic material. The water electrolysis device further includes a water inlet collector and an oxygen outlet collector, a straight line connecting the water inlet collector to the oxygen outlet collector extending along a general flow direction. The water electrolysis device further includes an anode facing the anode active layer. In a section along a normal to the straight line, the anode has points that each have a combined distance with the water inlet collector and with the oxygen outlet collector, the combined distance for a first point at the periphery of the anode is at least 10% greater than the combined distance of a second point positioned on the straight line.

The invention relates to electrolysers for the production of dihydrogen by electrolysis of water, and in particular the electrolysers equipped with proton-exchange membranes.

An electrolyser cell comprises a membrane-electrode assembly, with the proton exchange membrane equipped with an anode active layer on one face and with a cathode active layer on the other face. Water is introduced at the anode. Oxygen is produced at the anode by oxidation of water and protons are released. The protons pass through the membrane forming a solid electrolyte, and hydrogen is produced at the cathode by reduction of protons.

The anode materials must resist high potentials. Consequently, the number of electrocatalytic materials available to form an anode active layer is relatively small. The electrocatalytic materials of the anode layers are thus generally IrO₂ or alloys of IrO₂X IrO₂Rt.

Great disparities in current densities have been noted in various zones of the membrane-electrode assembly, even while trying to maintain a homogeneous pressure over the entire surface of the membrane-electrode assembly and while trying to homogenize as far as possible the electrical properties over this entire surface. It is possible in particular to observe a current density gradient between the centre of the anode and the periphery thereof. This current density gradient increases further with the ageing of the electrolyser. Such a gradient is prejudicial to the operation of the electrolyser, by dissociating several different operating regimes as a function of the zones of the anode. A concentration of the current density in the central part of the anode furthermore induces a premature degradation of the catalyst, due to a current density higher than that for which the anode was designed. Too low a current density at the periphery of the anode may on the contrary induce a chemical degradation of the proton-exchange membrane.

The invention aims to solve one or more of these drawbacks. The invention thus relates to a water electrolysis device, as defined in the appended claims.

The invention also relates to the variants of the dependent claims. A person skilled in the art will understand that each of the features of the dependent claims or of the description may be combined independently with the features of an independent claim, without however constituting an intermediate generalization.

Other features and advantages of the invention will become clearly apparent from the description that is given thereof below, by way of nonlimiting illustration, and with reference to the appended figures, in which:

FIG. 1 is a schematic cross-sectional view of an example of an electrolysis device incorporating a membrane-electrode assembly;

FIG. 2 illustrates a schematic front view of a first example of configuration of an anode;

FIG. 3 illustrates a schematic front view of a second example of configuration of an anode;

FIG. 4 is a schematic front view of an anode active layer for the anode from FIG. 2;

FIGS. 5 and 6 illustrate the distribution of the current across an anode for an electrolysis device according to an example of implementation of the invention, respectively during the activation thereof and after around a hundred hours of use;

FIG. 7 is a schematic cross-sectional view of a structure for a first example of a water diffusion element;

FIG. 8 is a schematic cross-sectional view of another structure for a second example of a water diffusion element.

FIG. 1 is a cross-sectional view of an example of a water electrolysis device 1 for producing dihydrogen. The electrolysis device 1 comprises an electrochemical cell 2 and an electrical power supply 3.

The electrochemical cell 2 comprises a membrane-electrode assembly 4, flow diffusion elements 5 and 6 positioned on either side of the membrane-electrode assembly 4, and electrical power supply plates 7 and 8. The plate 7 forms a cathode and the plate 8 forms an anode. The flow diffusion elements 5 and 6 are positioned respectively between the membrane-electrode assembly 4 and the cathode 7, and between the other membrane-electrode assembly 4 and the anode 8. The membrane-electrode assembly 4 comprises a proton-exchange membrane 43, and also a cathode active layer 41 and an anode active layer 42 fastened on either side of this proton-exchange membrane 43. The cathode 7 is positioned opposite the cathode active layer 41. The anode 8 is positioned facing the anode active layer 42.

A dihydrogen outlet collector 72 is made through one end of the cathode 7 and opens onto the diffusion element 5. The cathode 7 has a dihydrogen discharge line, not illustrated, in communication with the collector 72.

A water inlet collector 81 is made through one end of the anode 8 and opens onto the diffusion element 6. An oxygen outlet collector 82 is made through another end of the plate 8 and also opens onto the diffusion element 6. The supply plate 8 also has a role of guiding the flow of fluid between the collector 81 and the collector 82. A water supply line, not illustrated, is in communication with the collector 81. An oxygen discharge line, not illustrated, is in communication with the collector 82. A seal 9 is positioned between the anode 8 and the membrane-electrode assembly 4, level with the periphery of the anode 8. The seal 9 is intended to ensure the leaktightness of the flow between the collectors 81 and 82.

A first example of configuration of the anode 8 is illustrated with reference to FIG. 2. The anode 8 has here a substantially circular shape. A general flow direction is illustrated here by the dot and dash straight line X connecting the collector 81 and the collector 82. The direction of flow between the collectors 81 and 82 is illustrated here by the dotted line arrow.

A second example of configuration of the anode 8 is illustrated with reference to FIG. 3. The anode 8 has here a substantially rectangular shape.

The electrical power supply 3 is configured to apply a DC voltage generally between 1.48 V and 3 V, with a current density at the supply plates of between 500 and 40 000 A/m². By applying such a voltage, a water oxidation reaction at the anode 8 produces dioxygen, a proton reduction reaction at the cathode 7 producing dihydrogen.

The reaction at the anode 8 is the following:

2H₂O→4H⁺+4e⁻+O₂

The protons generated by the anode reaction pass through the proton-exchange membrane 43 up to the cathode 7. The power supply 3 conducts the electrons generated by the anode reaction to the cathode 7.

The reaction at the cathode 7 is thus the following:

2H⁺2e⁻→H₂

The proton-exchange membrane 43 has the role of being passed through by protons from the anode active layer 42 to the cathode active layer 41, while blocking the electrons and also the dioxygen and dihydrogen generated.

As illustrated in FIG. 2, the anode 8 is wider (the width is defined here as the maximum dimension of the anode 8 along a direction perpendicular to the flow direction X) than the collectors 81 and 82. If the flow between the collectors 81 and 82 is examined at various points along the width (various points in a section along a normal to the flow direction X), it is observed that the flow of fluid passing through these various points follows pathways that have quite different lengths. The pressure drop for these various pathways may have great variations, owing to the presence of water in these flows. This proves particularly pronounced halfway between the collectors 81 and 82.

This difference is illustrated in FIG. 2 by the points A and B, positioned on a diameter of the anode 8 perpendicular to the straight line X. Point B is positioned on a straight line connecting the collectors 81 and 82. Point A is positioned at the periphery of the anode 8. It is possible to determine the combined respective distances of these points from the collector 81 on the one hand and the collector 82 on the other hand. For point B, this combined distance DB is equal to the distance between the collectors 81 and 82. For point A, this combined distance DA is equal to the sum of the distance DA1 between point A and the collector 81 and of the distance DA2 between point A and the collector 82. The difference between the distances DA and DB is at least equal to 10%, and is approximately equal to 40% in the example illustrated. In FIG. 3, a point A and a point B have also been identified for the configuration of the anode 8 of substantially rectangular shape, also with differences between the distances DA and DB.

These disparities in distances for various points of the flow give rise to disparities in pressure drop, the pressure drop of the flow passing through point A being greater than that of the flow passing through point B. These differences in pressure drops lead to disparities in anode current, which may in particular result in an accelerated ageing of certain zones of the electrochemical cell 2. The invention proves particularly advantageous when there is a difference of at least 20% between the combined distances for two points of a same section.

In order to homogenize the current density across the anode 8 in various zones, the invention proposes to carry out a loading of the anode active layer 42 with electrocatalytic material that is at least 10% greater for a point facing the periphery of the anode 8 compared to a point positioned on the straight line X, these two points being positioned in the same section of the anode normal to the straight line X.

Thus, in the example from FIG. 2, the electrocatalytic material loading of the anode active layer 42 at point A is at least 10% greater than the loading thereof at point B. By increasing the electrocatalytic material loading in a zone in which the flow flux is lower, the current density can be homogenized over the whole of the surface of the anode 8.

In the example, a peripheral zone 85 of the anode 8 has for example been differentiated relative to a median zone 86. The border between the zones 85 and 86 is illustrated here by a dot and dash circle. FIG. 4 is a schematic front view of the active layer 42. The active layer 42 thus has a peripheral zone 425 superposed on the zone 85, and a median zone 426 superposed on the zone 86.

The surface loading of the anode active layer with electrocatalytic material in the zone 426 is at least 10% greater than the surface loading thereof in the zone 425. In the example illustrated, the surface loading with electrocatalytic material is homogeneous in the zone 425 of the active layer 42. The surface loading with electrocatalytic material is also homogeneous in the zone 426. The zone 426 may for example be produced on a strip of continuous width starting from the periphery of the active layer 42. The zone 426 may for example be sized so as to form at least 20%, preferably at least 35% of the surface area of the active layer 42.

It is also possible to envisage producing the active layer 42 with a disc-shaped median zone, a strip-shaped peripheral zone, and a strip-shaped intermediate zone, positioned between the disc and the peripheral strip. It is thus possible to better differentiate various operating zones of the active layer 42, with different surface loadings of electrocatalytic material. The limits of the peripheral strip and of the intermediate strip may be defined by a distance from the peripheral edge of the active layer 42.

The production of the membrane-electrode assembly 4 with an anode active layer 42 having zones with different electrocatalytic loadings may easily be carried out by means of spray inkjet printer. Such a printer may then be used to deposit a different amount of ink at various locations of a face of the membrane 43, with an ink that includes the electrocatalytic material in solution in an ionomer. The amount of ink may for example be adapted by adjusting the flow rate of ink in various zones or by using the same flow rate but by differentiating the number of layers deposited in superposition by the printhead for various zones. Such a process therefore easily makes it possible to adjust the surface loading of the layer 42 with electrocatalytic material. Via such a process, the thickness differences for the various zones of the layer 42 are negligible and do not impair the operation of the cell 2.

The electrocatalytic material of the anode active layer 42 may for example advantageously be IrO₂ or alloys of IrO₂X (X=Ru, Sn). The electrocatalytic material of the anode active layer 42 advantageously includes one of these materials at more than 50% by weight in the solids content of the catalytic layer. The mean surface loading of the anode active layer 42 with such an electrocatalytic material may for example be between 1 and 3 mg/cm², preferably between 1.7 and 2.1 mg/cm². Thus, with such loading, performances similar to those from the prior art would be obtained with reduced ageing and an electrocatalytic material cost that is also reduced.

In a manner known per se, the electrocatalytic material of the anode layer 42 may be coated with a polymer matrix, for example an ionomer such as a fluoropolymer based on sulfonated tetrafluoroethylene.

It is also possible to envisage that the electrocatalytic material of the layer 42 is fastened to a support such as antimony-doped tin oxide (ATO).

Advantageously, the anode active layer 42 is devoid of a carbon-based support.

The proton-exchange membrane 43 may for example be produced with a thickness at least equal to 100 μm. The membrane 43 may for example be made of an ionomer sold under the trade name Nafion.

Conclusive tests have in particular been carried out with various designs of anode active layers 42, for a layer 42 and an anode 8 that are circular.

For example, a first type of layer 42 was incorporated into a membrane-electrode assembly 4 with the following parameters:

-   -   a mean surface loading of the layer 42 with IrO₂ of 1.795         mg/cm²;     -   a peripheral strip of the layer 42 having a mean surface loading         with IrO₂ of 1.98 mg/cm²;     -   a median disc of the layer 42 having a mean surface loading with         IrO₂ of 1.25 mg/cm²;     -   an intermediate strip of the layer 42, positioned between the         peripheral strip and the median disc, having a mean surface         loading with IrO₂ of 1.56 mg/cm²;     -   a distribution of electrocatalytic material of 30% on the         peripheral strip, 62% on the intermediate strip and 8% on the         median disc;     -   a cathode active layer 41 that includes a mean surface loading         with Pt on a carbon-based support of 0.8 mg/cm².

According to another example, a second type of layer 42 was incorporated into a membrane-electrode assembly 4 with the following parameters:

-   -   a mean surface loading of the layer 42 with IrO₂ of 2.046         mg/cm²;     -   a peripheral strip of the layer 42 having a mean surface loading         with IrO₂ of 2.5 mg/cm²;     -   a median disc of the layer 42 having a mean surface loading with         IrO₂ of 1.4 mg/cm²;     -   an intermediate strip of the layer 42, positioned between the         peripheral strip and the median disc, having a mean surface         loading with IrO₂ of 1.87 mg/cm²;     -   a distribution of electrocatalytic material of 30% on the         peripheral strip, 62% on the intermediate strip and 8% on the         median disc;     -   a cathode active layer 41 that includes a mean surface loading         with Pt on a carbon-based support of 0.8 mg/cm².

FIGS. 5 and 6 illustrate the distribution of the current across an anode 8 associated with an active layer 42 according to the second type, respectively during the activation thereof and after around a hundred hours of use. Various ranges of current density in A/cm² are illustrated here.

It can be noted that the invention has made it possible to obtain a relatively homogeneous current density for the various zones, namely the median zone, the intermediate strip and the peripheral strip. It can therefore be assumed that the ageing of the various zones of the membrane-electrode assembly will also be homogeneous. The invention makes it possible in particular to prevent the concentration of the current in the median zone. The invention also makes it possible to increase the current density for the peripheral zone, which makes it possible to make good use of a zone of the active layer 42 forming the major part of its surface but that usually has a lower performance due to the pressure drops that the flow of fluid is subjected to in order to arrive thereat. The mean current density at the activation rose to 0.405 A/cm².

After ageing, it is observed that the homogeneity of the distribution of the current density at 0.397 A/cm² is not impaired. Which demonstrates the advantage of the structuring of the active layer in order to minimize the degradation phenomena of the MEA.

The electrochemical cell 2 advantageously comprises a flow diffusion element 6 positioned between the membrane-electrode assembly 4 and the anode 8.

FIG. 7 is a schematic cross-sectional view of a structure for a first example of a water diffusion element 6. The water diffusion element 6 here comprises a superposition of several screens 61 to 63 and of a porous layer 64. The porous layer 64 is positioned in contact with the layer 42, the screen 61 being placed in contact with the anode 8. In this example, three screens 61 to 63 have been superposed. The screens 61 to 63 have relatively wide meshes, with a width at least equal to 1 mm.

The screens 61 to 63 have the role of distributing the water within the volume. The porous layer 64 may for example have pores having a size of between 10 and 50 μm, with a porosity of between 30 and 50%. The porous layer 64 is intended to enable a fine distribution of water to the layer 42. The porous layer 64 is further intended to discharge the gases produced, and to enable an electrical contact that is as homogeneous as possible. It can also be envisaged to superpose several porous layers 64 with different porosities, in order to create a porosity gradient. The screens 61 to 63 and the porous layer 64 are electrically conductive. The thickness of this water diffusion element 6 may for example be between 1 and 3 mm.

FIG. 8 is a schematic cross-sectional view of another structure for a second example of a water diffusion element 6. The water diffusion element 6 is here in the form of a water flow guide. The element 6 is formed from a single part. The element 6 comprises walls 64, delimiting flow channels 63. The flow channels 63 are parallel to the straight-line X, and thus make it possible to guide a flow of fluid from the collector 81 to the collector 82. The depth of the flow channels 63 is for example between 1 and 3 mm. 

1: A water electrolysis device, comprising: a membrane-electrode assembly, comprising a proton-exchange membrane and an anode active layer that includes an electrocatalytic material and is positioned on one face of said proton-exchange membrane; a water inlet collector; an oxygen outlet collector, a straight line connecting the water inlet collector to the oxygen outlet collector extending along a general flow direction; an anode facing the anode active layer, having one end in communication with the water inlet collector and another end in communication with the oxygen outlet collector; characterized in that: in a section along a normal to said straight line, the anode has points that each have a combined distance with the water inlet collector and with the oxygen outlet collector, the combined distance for a first point at the periphery of the anode being at least 10% greater than the combined distance of a second point positioned on said straight line; the surface loading of the anode active layer with electrocatalytic material at the first point being at least 10% greater than the surface loading thereof at the second point. 2: The device according to claim 1, in which the electrocatalytic material of the anode active layer includes IrO₂ at more than 50% by weight in the solids content of this anode active layer. 3: The device according to claim 1, in which the electrocatalytic material of the anode active layer includes IrO₂X with X=Ru or Sn, at more than 50% by weight in the solids content of this anode active layer. 4: The device according to claim 2, in which said anode active layer includes a mean surface loading of IrO₂ of between 1 and 3 mg/cm². 5: The device according to claim 1, in which said proton-exchange membrane has a thickness at least equal to 100 μm. 6: The device according to claim 1, in which the combined distance for the first point (A) is at least 20% greater than the combined distance for the second point (B). 7: The device according to claim 1, in which the anode has a circular shape. 8: The device according to claim 1, in which said anode active layer includes a polymer matrix in which the electrocatalytic material is coated. 9: The device according to claim 1, further comprising a permeable water diffusion element positioned between the anode active layer and the anode. 10: The device according to claim 9, in which said permeable element comprises a stack of several screens and of a porous layer. 11: The device according to claim 1, comprising a flow guide positioned between the anode and the anode active layer, the flow guide defining flow channels between the water inlet collector and the oxygen outlet collector. 12: The device according to claim 1, in which said anode active layer is devoid of a carbon-based support. 13: The device according to claim 1, further comprising: a cathode active layer positioned on one face of said proton-exchange membrane on the opposite side to the anode active layer; a cathode facing the cathode active layer, the cathode and the anode being electrically conductive; a power supply circuit configured to apply potential difference between the anode and the cathode. 